1. Field of the Invention
The present invention relates generally to an apparatus and method for preventing disbondment in a bonded foam insulated piping system of the type used for conveying high temperature fluids.
2. Description of the Prior Art
Insulated pipelines are needed in a variety of situations. For example, distributed HVAC (heating, ventilation, and air conditioning) applications utilize chilled water for cooling and steam for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam are distributed to other locations. For example, on a school campus the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings. A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulted pipelines is used to carry the steam from the boiler to the other locations and back to the boiler. Oftentimes, the temperature inside the pipe is either higher or lower than the ambient temperature surrounding the pipe. It is necessary for the pipes to be insulated in order to retain the internal temperature of the fluids and keep heating and cooling losses at a minimum. The insulated pipelines are usually located underground.
Insulated pipe of the type under consideration is conventional and commercially available. There are predominately two types of piping systems in use: Class-A drainable dryable testable (DDT); and polyurethane or polyisocyanurate bonded foam systems. The present application is directed toward the bonded foam type system. These systems utilize a steel pipe to convey fluid, and often the fluid is a different temperature as compared to the ambient environment. Around the outside of the steel pipe is a layer of insulating foam such as, for example, polyisocyanurate foam. In the case of high temperature piping systems, the insulating foam serves to keep heat loss from the starting location of the pipeline to the ending location at a minimum. Around the outside of the foam is a thin jacket of thermoplastic material, such as high density polyethylene (HDPE). The plastic jacket protects the foam from mechanical damage and also provides a watertight seal to prevent corrosion of the steel pipe. Although steel is commonly used for the inner pipe which carries the media to be piped, copper, aluminum or other metals as well as fiberglass, PVC, and similar materials may be utilized, as well.
The most important engineering criteria for a foam system of the type under consideration is that it must be treated as a bonded system. In other words, the foam is bonded to both the carrier pipe and the outer jacket. In such a case, the bonded system acts as a monolithic unit moving underground. Higher temperatures can act adversely upon the bonded foam system, however. The hot fluid in the steel carrier pipe causes the carrier pipe to thermally expand. At temperatures of 400° F. this expansion is on the order of 2.8 inches per 100 feet of pipe. This expansion is not a problem as long as the system remains bonded and the carrier pipe, foam and jacket move together as one unit underground. This movement is controlled by the expansion force of the steel carrier pipe, but it is the bond strength of the foam to the pipe and jacket that is important in keeping the system moving together. This monolithic movement of the system occurs along each incremental length of a particular run, and as long as total movement is not greater than 4 to 6 inches and the system remains bonded, no undue stress is subjected at any one point of the jacket. If the system were to disbond, however, the surrounding soil would fix the jacket in place and the carrier pipe would still thermally expand thereby pushing through and destroying the jacket at the first change of direction.
Generally speaking, the proper choice of insulating materials can counteract many of the thermal expansion effects discussed above. It has been well established by industry case history that the polyurethane foam bond for systems running at 250° F. to 300° F. is strong enough to keep the entire system acting as a bonded system. However, for systems running above these temperatures a higher temperature rated foam, such as polyisocyanurate foam, is generally required. Even in systems utilizing “high temperature” polyisocyanurate foam, the higher heat can, in some instances, begin to fry the foam at the foam/carrier pipe interface, thereby bringing into question the strength of the foam bond to the steel carrier pipe.
Various approaches have been taken to control this undesirable expansion in insulated pipe systems of the type under consideration. For example, expansion “bolster” materials are supplied in the form of resilient pads which can be used at elbows or expansion loops. These pads are placed adjacent to the piping and create a cushion which acts as a stress relief area in critical areas, such as angles and elbows.
Despite the advances seen in the high temperature piping industry, a need continues to exist for improved systems for preventing disbondment in bonded foam insulated piping systems.
A need also exists for such an improved system which utilizes many of the conventionally available materials and manufacturing techniques commonly used in the industry.
A need also exists for such a system which is simple in design and economical to implement.